Glucokinase (GK, Hexokinase IV) is one of four hexokinases that are found in mammals (Colowick, S. P., in The Enzymes, Vol. 9 (P. Boyer, ed.) Academic Press, New York, N.Y., pages 1-48, 1973). The hexokinases catalyze the first step in the metabolism of glucose, i.e., the conversion of glucose to glucose-6-phosphate. Glucokinase is found principally in pancreatic β-cells and liver parenchymal cells, two cell types that are known to play critical roles in whole-body glucose homeostasis. Specifically, GK is a rate-controlling enzyme for glucose metabolism in these two cell types (Chipkin, S. R., Kelly, K. L., and Ruderman, N. B. in Joslin's Diabetes (C. R. Khan and G. C. Wier, eds.), Lea and Febiger, Philadelphia, Pa., pages 97-115, 1994).
The concentration of glucose at which GK demonstrates half-maximal activity is approximately 8 mM. The other three hexokinases are saturated with glucose at much lower concentrations (<1 mM). Therefore, the flux of glucose through the GK pathway rises as the concentration of glucose in the blood increases from fasting levels (5 mM) to postprandial levels following a carbohydrate-containing meal (about 10-15 mM) (Printz, R. G., Magnuson, M. A., and Granner, D. K. in Ann. Rev. Nutrition Vol. 13 (R. E. Olson, D. M. Bier, and D. B. McCormick, eds.), Annual Review, Inc., Palo Alto, Calif., pages 463-496, 1993). These findings suggest that GK functions as a glucose sensor in β-cells and hepatocytes (Meglasson, M. D. and Matschinsky, F. M. Amer. J. Physiol. 246, E1-E13, 1984).
More recently, studies in transgenic animals confirmed that GK does indeed play a critical role in whole-body glucose homeostasis. Animals that do not express GK die within days of birth with severe diabetes, while animals overexpressing GK have improved glucose tolerance (Grupe, A., Hultgren, B., Ryan, A. et al., Cell 83, 69-78, 1995; Ferrie, T., Riu, E., Bosch, F. et al., FASEB J., 10, 1213-1218, 1996). An increase in glucose exposure is coupled through GK in β-cells to increased insulin secretion and in hepatocytes to increased glycogen deposition and perhaps decreased glucose production.
The finding that type II maturity-onset diabetes of the young (MODY-2) is caused by loss of function mutations in the GK gene suggests that GK also functions as a glucose sensor in humans (Liang, Y., Kesavan, P., Wang, L. et al, Biochem. J. 309, 167-173, 1995). Additional evidence supporting an important role for GK in the regulation of glucose metabolism in humans was provided by the identification of patients that express a mutant form of GK with increased enzymatic activity. These patients exhibit a fasting hypoglycemia associated with an inappropriately elevated level of plasma insulin (Glaser, B., Kesavan, P., Heyman, M. et al., New England J. Med. 338, 226-230, 1998). Accordingly, compounds that activate GK and, thereby, increase the sensitivity of the GK sensor system are expected to be useful in the treatment of the hyperglycemia characteristic of all type II diabetes. Glucokinase activators should increase the flux of glucose metabolism in β-cells and hepatocytes, which will be coupled to increased insulin secretion.
There is a continued need to find new therapeutic agents to treat human diseases. The hexokinases, specifically but not limited to glucokinase, are especially attractive targets for the discovery of new therapeutics due to their important role in diabetes, hyperglycemia and other diseases.